1. Field of the Invention
The present invention generally pertains to electrically conductive bracelets worn on the ankle or wrist to drain or wick static electrical charge from the wearer. The present invention specifically concerns stretchable elastic fabric bracelets with an insulating surface upon the external circumference of the bracelet.
2. Description of the Relevant Art
As reported in U.S. Pat. No. 4,577,256, static electricity causes problems in the electronics and other industries, particularly with the advent of integrated circuits and other microelectronic components. Components such as integrated circuits, for instance, may be disabled or destroyed by over-voltage or power density resulting from static electricity. Certain junctions in such circuits can be destroyed by as little as a 50-volt potential, which radically changes the doping structure in their lattices. Power densities resulting from excessive potential and imperfections in a silicon circuit layout or structure can vaporize or radically alter the silicon substrate and thus impair or destroy a circuit's performance. Yet a person walking on a carpet on a dry day can accumulate as much as 30,000 volts of potential, and he can triboelectrically generate thousands of volts by simply changing his position in his chair or handling a styrofoam cup.
Such a person can inadvertently discharge static electric potential into a circuit or component by touching it and causing overvoltage or excessive power density. Additionally, the potential in such a person's body can induce a charge in a circuit that can later cause overvoltage or excessive power density when the circuit is subsequently grounded. More and more frequently, therefore, personnel in industries in which integrated circuits and other microelectronic components are handled or assembled are taking measures to limit the failure rate of those circuits and components by attempting to keep both themselves and their environment at a zero electrical potential. Such measures include providing workers and work stations with antistatic carpet; conductive or dissipative grounded desk top work surfaces; hot air ion generators which emit ions to neutralize static charges; and grounding straps worn by workers to keep workers at zero potential. The term "conductive" herein, and according to its customary usage in the art, means an electrical resistance of between zero and 10.sup.5 ohms. Similarly, "dissipative" means a resistance of between 10.sup.5 and 10.sup.9 ohms, "anti-static" means a resistance of between 10.sup.9 and 10.sup.14 ohms, and "insulative" means a resistance of more than 10.sup.14 ohms.
With specific relation to grounding strap measures grounding strap must have several features in order to perform its grounding function effectively. First, it must ensure that the wearer's skin is electrically connected to ground. This connection is typically accomplished by a conductive surface on the inside of a strap contacting the skin. The conductive surface is electrically connected to a grounding cord which leads from the strap to a grounded electrical connection. If the electrical contacting means on the inside of the strap becomes dirty or fouled by oil, perspiration or hair, then the strap may lose its effectiveness. It is therefore important to use a conductive material on the inner surface of the strap that does not easily become dirty or fouled.
Secondly, user comfort is a premier consideration because if the strap is uncomfortable, then the wearer will be tempted to remove it and can thereby cause damage to electrical components on which he is working. A strap that is easily stretchable, that is attractive and that poses minimum inconvenience to the wearer is therefore highly desired.
The situations in which grounding wrist straps are used heightens the importance of their being comfortable so that they are continuously worn and maintain continuous electrical contact with the skin. A person working on microelectronic components or integrated circuits may be completely unaware that he has accumulated minor static electrical discharges, and may therefore unknowingly be in a position to disable circuits on which he is working or which he is handling. If his strap is loose or he has removed it, he may be unaware that electrical discharges transmitted from his fingers are disabling these circuits. (A typical person cannot sense a static electrical discharge of less than approximately 3,500 volts.) No one may discover that the circuits have been disabled or damaged until hours, days or weeks later, when the circuits have been placed in components or devices which fail in the field. Removal and repair or replacement of these circuits once in the field is far costlier than avoiding potential failure while the wearer is handling the circuits. Thus, a manufacturer typically must depend upon the effectiveness of the wrist strap to maintain a lower failure rate of such electronic circuits and components, by maintaining continuous electrical contact with the wearer's wrist and by providing minimum temptation to remove the strap from his wrist.
Finally, grounding wrist straps should be electrically insulating on their exterior surfaces (at least in all areas wherein electrical connection is not intentionally made for the wicking of electrical charge to ground) in order that the hazard to the wearer from inadvertant contact of the bracelets surface with a source of electrical potential should be reduced. The interior circumferential surface of a grounding strap is intentionally made to be an excellent electrical conductor and be incontact with the user. If the bracelet's outer surface were to be conductive, and were in electrical contact with the inner circumferential surface, then an efficacious, and hazardous, path of electrical current conduction to the body might be presented. This path would generally be of lower resistance that that presented by the naked skin. Furthermore, if the bracelet's outer surface were to catch or snag on a source of electrical energy then a wearer might be detrimentally placed in contact with this source for a longer period of time than would otherwise be the case.
These considerations of selective conductivity and of comfort have been recognized, and have been addressed, by several types of grounding straps. A first type of grounding straps are made of metal. U.S. Pat. No. 4,373,175 issued Feb. 8, 1983 to Mykkanen, for instance, discloses an extensible metal band similar to an expansion watchband on which a snap fastener for a grounding cord is attached. Such a strap can be reasonably comfortable. However, its conductive metal outer surface can prove dangerous to the wearer if it contacts an electrical potential sufficient to electrocute the wearer.
U.S. Pat. No. 4,402,560 issued Sept. 6, 1983 to Swainbank discloses an expansive metal conductive wrist strap improved for having a resistor, enclosed within a housing, in the ground lead at a point proximate the point of plugged electrical connection to the wrist strap. The housing is adapted to be grasped by the fingers for plugging and unplugging the ground lead from the wrist strap.
Another, second, type of prior art grounding straps is made of textile material In one such all-textile strap the textile is impregnated with a thermosetting conductive coating and fastened about the wrist of an operator. The fabric grounding strap is connected by way of a swivel type snap connector and insulated conductor to a suitable device for making a connection to ground at the work station.
One fabric grounding strap reportedly improved for comfort is disclosed in U.S. Pat. No. 3,857,397 to Brosseau. Outer and inner conductive polyolefin layers sandwich an intermediate nylon scrim layer to form the band. Hook and loop (Velcro .RTM.) fastening materials can accumulate on such surfaces and interfere with electrical contact between the band and the skin. Further, carbon particles tend to wear off onto the wrist, causing black stripes on the wrist. The nonstretchable nature of such bands means that the wearer must adjust then to be tight enough to cause sufficient elecrical contact, but loose enough to be comfortable, and skin contact can be lost or intermittent.
Another approach is disclosed in U.S. Pat. No. 4,398,277 to Christiansen and Westberg ("Christiansen"). This strap is made of knitted stretchable fabric containing stainless steel fibers. A plastic and metal fitting permanently closes the strap into a loop of predetermined size and also has a connection for a grounding cord. This strap can prove uncomfortable to the wearer, however, unless his wrist comports with the predetermined strap size offered by the manufacturer. Further, the knitted fabric permits the strap to roll over on itself as it is being pulled over the hand and causes the strap to become thinner as it is stretched. Because the fabric is knitted, it can also "pull" and "run" when snagged. Perhaps more important it has been discovered that the electrical conductivity of the Christiansen strap decreases as the strap is relaxed, and thus varies from one stretched state to another. This phenomenon probably occurs because the metallic strands in the conductive yarns are pulled more tightly together in the knitted material as it is stretched, and are separated from one another to a certain extent in the knitted conductive yarns as the strap is relaxed.
A reported solution to some of these problems with grounding wristbands made of fabric is discussed in U.S. Pat. No 4,577,256 to Breidegam. A woven stretchable grounding strap is shown which uses conductive fibers on the inside surface of the strap to contact the skin and to conduct electrical charges to a grounding cord attached to the strap. Face yarns exposed on the outer surface are woven to form designs. The woven fabric material of the strap is attached to a clasp allowing the strap to be adjustable in size. Because of the woven nature of the fabric material and the adjustable clasp, the strap is reported to be more comfortable than other conductive elastic wrist straps. The woven fabric material is also reported to be advantageous because it stretches easily, it is relatively inexpensive, and it does not roll over onto itself as it is being drawn over the hand. This woven stretchable grounding strap is in actual industrial usage circa 1987.
This prior art fabric wristband described in the Breidegam patent--which is visually similar to the fabric elastic wrist bracelet in accordance with the present invention--has been found, however, to be subject to latent defects, causing failure of function, during manufacture and use. Specifically, the construction of the Breidegam prior art wristband by the process of weaving innately requires that the conductive thread, or yarn, which is necessary for the wristband's conductive function should be interlaced to and extend through, the inner and the outer, sides of a fabric body, or garter. Because electrical conductivity is desired upon only one side (the inner side) of the wristband, additional face yarn is applied over the outer side in order to prevent the conductive yarn from emerging. The effectiveness of insulating one layer of electrically conductive yarn by an overlying layer of another, electrically insulating yarn, during manufacture has proven to be poor and unreliable. Worse, since the wristband is necessarily stretched and contracted during fitting and use, the face yarn loosens and shifts thereby allowing the conductive yarn to progressively intermittently emerge in sporadic areas. Because this emergence which produces increased conductivity at the outer surface of the wristband has no detrimental effect on the wristband's primary conductive function to wick electrical charge, it may go unnoticed and/or ignored by the wristband user. However this latent increase in conductivity of the wristband's outer surface can have severe safety consequences if, and when, the supposed insulating qualities of the outer surface are ever called upon to protect the user from electrical shock. Since periodic electrical testing of fabric grounding straps for their insulative, as opposed to their conductive, properties seems to be cumbersome, of uncertain efficacy, and incompatible with the in-line use and use environment of the straps, it is highly desirable that the problem of latent outer surface conductivity should be definitively solved, and not merely recognized in order that it might be abated.